Wire bonding is used in the microelectronics industry as a means of making electrical interconnections between semiconductor chips and substrates (such as leadframe carriers and printed circuit or wire boards). Wires made from gold, aluminum and copper are commonly used. The interconnect process is automated and it is important to develop processes that are of low cost and high yield, with sufficient long-term reliability to satisfy an end-user.
With improvements in back-end semiconductor technology and techniques, bonding wire diameter and bond pitch are becoming smaller and packaging density is on the increase. As a result, greater accuracy and stability is required to form well-controlled wire loops. These wire loops need to be inspected to ensure that they are properly formed within certain pre-defined parameters. By practicing height measurement, stability and reliability of a bonding machine may be maintained. To conduct inspection and measurement, optical methods are usually preferred because they are non-destructive and non-intrusive.
There are various methods used in the prior art to measure a height of a wire loop. One example is U.S. Pat. No. 4,942,618 for “Method and Apparatus for Determining the Shape of Wire or Like Article”. This method employs a plurality of coaxial CCD cameras having their focal planes positioned differently from each other to view a wire loop. The wire loop is viewed by the CCD cameras and the wire images collected by the plurality of cameras are processed to determine the contrast and size of the images, and the results are used to determine the profile of the wire. However, such an apparatus is bulky and cumbersome to control. Further, more cameras have to be used to improve measurement resolution and accuracy.
Another example is U.S. Pat. No. 5,576,828 for “Bonding Wire Detection Method”. In this disclosure, the height of a wire bonded between a semiconductor chip and a lead frame is determined using illumination from circularly arranged LED's installed in a low-angle illuminating device. The angle of illumination is set within a certain range and a focal depth of an optical device is set to be shallow. A dark area appears in the central portion of the wire at the focal point of the optical system. The optical device is moved up and down to obtain a focused image of the dark area, so as to determine the height of the wire relative to a height of the optical system. A problem with this apparatus is that the imaging system is bulky. As a result, the need to lift and lower the relatively large mass of the optical system results in reduced speed of motion on the whole.
A further example of the use of an optical system to measure a height of a wire loop is disclosed in U.S. Pat. No. 5,583,641. A correlation between a wire height and wire width is investigated by shifting the focusing level of an optical system. A detection level is set based upon an upper limit level and a lower limit level. These levels are to be the reference levels corresponding to an acceptable height of the wire to be detected. The imaged width of the wire is compared to the upper and lower limit levels to determine whether the height of the wire is within an acceptable range. A problem with this system is again that the optical system or a part of it has to be moved up and down to measure different points on the wire, and the motion of such a relatively large mass slows down the apparatus. The imaging system is also bulky, and the optics design is relatively complex.